(a) Field of the Invention
This invention relates to a process for the nitration of 2-methylimidazole by means of nitric and sulphuric acid, assuring full safety and high output, notwithstanding the rapid and intensive course of the reaction as well as the use of high concentrated acid solutions.
(b) Prior Art
It is very difficult to introduce the nitric group into the imidazole ring because this requires the employment of extremely intense conditions, i.e. high temperature as well as high excesses of concentrated acids, such as nitric and sulphuric acid. This reaction is accompanied with a very intensive development of nitrogen oxides and a very high exothermic effect coupled with a splitting of part of the 2-methylimidazole and a full oxidation of the split products. By such conditions, the completion of this process, which runs effectively on the edge of a nitration and a splitting producing great quantities of energy, is indeed difficult, since at each moment any uncontrolled, violent reaction can occur what can lead to an explosion.
As industrial conditions for carrying out this reaction it is fundamentally necessary to provide assurance of complete safety, and therefore when employing known processes for the nitration of 2-methylimidazole, special methods of proceeding have been used. These consisted of first forming the solution of the 2-methylimidazole in sulphuric acid with the formation of an acid sulfate of the 2-methylimidazole.
This solution has been added in small portions or dropped very slowly into the nitrating mixture in order to have in the reacting solution only small quantities of 2-methylimidazole or to assure a full reaction before the introduction of each new portion. There were also employed reverse methods consisting in a slow and gradual introduction of nitrates or of nitric acid into the solution of 2-methylimidazole in sulphuric acid. In any event, the principle of such a processing is to keep in the solution during the nitration only small quantities of one of the reacting agents, i.e. of the organic substance or of the eventual oxidizer. Such a method could be used under industrial conditions, but the slow operation of this process considerably increases the processing costs and notwithstanding these safety measures, there is still no guaranty of a safe course of the reaction and there have occurred accidents and even explosions. It has been found through research, that the cause of these accidents was that in the presently used processes, the nitration of the sulphate of 2-methylimidazole does not start at once after the introduction of the reagents, but requires an induction time, which unfortunately is variable and difficult to be predicted. Owing to this, when reagents are continuously added, they can accumulate in the solution and cause an explosion. The velocity of the reaction after the induction period indeed increases very quickly by producing a violent exothermic effect and the induction agents, which start the reaction, were unknown. Moreover, in the known methods, the output was generally relatively low, taking into consideration the great inconvenience caused by the necessity of applying special conditions during the operation of the process, and under the best conditions the output is only 48-52%.
Better results are obtainable by the known Pyman method, but this unfortunately is only suitable on a laboratory scale. This method consists of the nitration of very small portions, on the order of several grams of 2-methylimidazole, which were previously dissolved in nitric acid to yield the nitrate of this compound. To such a prepared compound, sulphuric acid was added; after a short period of time a rapid increase of temperature occurred and a violent reaction started, which by reason of the small quantities of reagents introduced in the reaction was not so dangerous. Because of such "microexplosions" it was possible to treat only small portions of this compound, but the use of this method on an industrial basis proved to be impossible. This method however obtained an output of 63% of theoretical.
Owing to the great demand for 2-methyl-5-nitroimidazole, a compound used as raw material in the chemical and pharmaceutical industry, it has been also proposed to make the nitration process of the 2-methylimidazole safer by employing, during the reaction, deterrents, i.e. neutral substances, which not only do not take active part in the nitration process, but dilute the reaction mixture and increase the mass and the thermal capacity of the system, which reduces the sensitivity of the mixture and the possibility of explosion, and in any event it reduces its energy.
Hence, according to USSR patent specification No. 20247, it was proposed to use a deterrent, an additional product introduced into the mixture, such as sodium sulphate with sulphuric acid or an aqueous solution of acid sodium sulphate.
In the process described in German Pat. No. DBP-1.808.104 and French Pat. No. 2.022.956it has been proposed to use as deterrents, carbamide and sulphuric acid or carbamide sulphate. The addition of the deterrent has rendered the operation of the reaction safer, but a serious disadvantage of this is the introduction of supplementary raw materials and the necessity of an important increase of nitrating acids already used in very high quantities, whereby owing to the dilution of the reaction product it was moreover necessary to carry out a troublesome process of isolating the reaction product from the large volume of the reaction mixture. Furthermore, the presence of foreign products made the following treatment very difficult, particularly the cleaning of the 2-methyl-5-nitroimidazole, which considerably increased the costs of production of this compound.